Polymers of vinyl ether and processes to prepare these polymers by cationic polymerization are known. For example, Higashimura describes a process for polymerizing vinyl ether and para-methoxy styrenes wherein a catalyst obtained by combining HI and I.sub.2 or ZnX.sub.2 wherein X is a halogen such as iodine, chlorine or bromine is used, Higashimura et al., Macromolecules, Vol. 12, p. 178 (1979). This system is effective in the preparation of these polymers, but the zinc halides are toxic and a process which utilizes a non-toxic catalyst is desirable. The zinc catalysts are also soluble in the polymer solutions and will contaminate the polymer requiring a costly catalyst extraction procedure. Without removing the catalyst the polymer will be highly disclored and the polymer generally has an undesirable color even after catalyst extraction.
Alternative processes to prepare vinyl ether polymers include utilization of zeolites as catalysts. Exemplary processes are described in, for example, U.S. Pat. Nos. 3,228,923; 3,365,433; 3,394,116; 3,461,075; and 3,819,596. Although the mechanism for this polymerization is not clear, it does not appear to be a cationic polymerization. The resultant polymers may have high molecular weights, but have an extremely wide molecular weight distribution. The polymer made by the process of U.S. Pat. No. 3,819,596 is touted as an excellent plasticizer due to the broad molecular weight distribution. The index of dispersity (ratio of highest molecular weight to peak molecular weight) for these polymers exceeds 20. The polymer produced by this method is said to have a molecular weight distribution which extends from 180 to 800,000. This wide molecular weight distribution is beneficial for some properties and end uses, such as tackifiers and plasticizers. For other end uses, a narrow molecular weight distribution is preferred. For molding compositions, films and extruded products, having a high tensile strength with a low melt viscosity is desirable. Compositions which have a wide variety molecular weights generally have a of high melt viscosity due to the high molecular weight constituents, and a relatively low tensile strength due to the low molecular weight constituents. A polymer with a narrow molecular weight distribution is therefore preferred for many applications.
Polymers which have narrow molecular weight distributions are also preferred as lubricating oil additives viscosity index improvers. Polymers that are high in molecular weight are degraded by mechanical shear while in lubricating oil service. High molecular weight polymers also contribute disproportionately to the thickening effect, or change in viscosity for a constant amount of polymer added. The shearing of these high molecular weight polymers therefore drastically affects the viscosity of the lubricating oil composition. Having a narrow molecular weight distribution therefore results in a viscosity index improver with a minimal change in viscosity over time for a similar initial thickening effect.
It is therefore an object of this invention to provide a process to polymerize vinyl ethers which results in polymer compositions having narrow molecular weight distributions and wherein soluble cocatalyst is not required.